1. Field of the Invention
The present invention relates to a code division multiplex signal transmitter, and more particularly to a code division multiplex signal transmitter for use on a transmitting end in a code division multiplex communication system. The present invention also relates to a code division multiplexing method therefor.
2. Description of the Background Art
A telecommunications network system relying upon the code division multiplexing (CDM) allows several pieces of transmission data to be multiplexed into one time slot. The CDM technique may, when applied to a telecommunications system, advantageously transmit large volumes of data with communications resources such as time slot saved, thus being put to practical use mainly in mobile communications.
Now, with a conventional CDM signal transmitter disclosed by, for example, U.S. Pat. No. 7,342,909 to Matsuno et al., a CDM signal transmitter is supplied with transmission data on a plurality (N) of channels, which will in turn be encoded by corresponding plural encoders with codes assigned to the respective channels. The resultant coded transmission data, which are binary signals having value “0” or “1”, are fed to respective modulators corresponding in number to the channels. The modulators modulate in amplitude the coded transmission data to thereby produce modulated signals. In the conventional CDM signal transmitter, the degrees of modulation are equally defined between those modulators so that the amplitude levels of the modulated signals are equal to each other. The modulated signals are then delivered to a multiplexer, which in turn multiplexes the modulated signals to produce a CDM signal, which is an (N+1)-valued signal that takes on any one of values 0, 1, 2 . . . and N.
However, the conventional CDM transmitter requires the modulators corresponding in number to the channels of transmission data. Thus, the number of modulators increases with an increase in number of channels, resulting in an increase in cost for required parts and components. Moreover, the circuit size of the multiplexer increases, so that the manufacture of multiplexer becomes difficult.